Group iii nitride semiconductor element and epitaxial wafer

ABSTRACT

A primary surface  23   a  of a supporting base  23  of a light-emitting diode  21   a  tilts by an off-angle of 10 degrees or more and less than 80 degrees from the c-plane. A semiconductor stack  25   a  includes an active layer having an emission peak in a wavelength range from 400 nm to 550 nm. The tilt angle “A” between the (0001) plane (the reference plane S R3  shown in FIG.  5 ) of the GaN supporting base and the (0001) plane of a buffer layer  33   a  is 0.05 degree or more and 2 degrees or less. The tilt angle “B” between the (0001) plane of the GaN supporting base (the reference plane S R4  shown in FIG.  5 ) and the (0001) plane of a well layer  37   a  is 0.05 degree or more and 2 degrees or less. The tilt angles “A” and “B” are formed in respective directions opposite to each other with reference to the c-plane of the GaN supporting base.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. patent application Ser. No.12/965,633 filed on Dec. 10, 2010, which is a continuation applicationof U.S. patent application Ser. No. 12/779,769 filed on May 13, 2010,which is a continuation application of a PCT application No.PCT/JP2009/056978 filed on Apr. 3, 2009, claiming the benefit ofpriorities from Japanese Patent application No. 2008-099625 filed onApr. 7, 2008, and incorporated by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a group III nitride semiconductordevice and an epitaxial wafer.

BACKGROUND ART

Patent Literature 1 discloses a semiconductor light-emitting device. Thesemiconductor light-emitting device has a semiconductor stack, whichlowers threshold current density for laser oscillation and reduces theoccurrence of kink, and the semiconductor light-emitting device of asurface-emitting semiconductor laser has a fixed polarization plane oflaser oscillation, and reduces a variation in the oscillation plane.

Patent Literature 2 discloses a light-emitting diode device, asemiconductor laser device, a photosensitive device and a transistorwhich are made of group III nitride semiconductors. These group IIInitride semiconductor devices include an AlGaN layer having a high Alcomposition and a high carrier concentration. The growth of the AlGaNlayer is carried out so as to enhance the surface diffusion of Al atoms,although the surface diffusion is small in AlGaN growth. The AlGaN layerwith a high Al composition and a high carrier concentration can be grownon the surface of a GaN substrate that tilts by an angle of 1 degree to20 degrees.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Unexamined Patent Application    Publication No. 10-135576-   Patent Literature 2: Japanese Unexamined Patent Application    Publication No. 2002-16000

SUMMARY OF INVENTION Technical Problem

In Patent Literature 1, an active layer is composed of a semiconductorhaving uniaxial anisotropy, and the thickness direction of the activelayer is different from the direction of the axis of the uniaxialanisotropy. The semiconductor light-emitting device is formed on asubstrate having a primary surface of (11-20) plane or (1-100) plane.The primary surface of the substrate can tilt from those planes by anoff-angle of zero degrees or more, and the upper limit of the tilt angleis 10 degrees in order to prevent, for example, formation of a twincrystal. More specifically, the primary surface is an a-plane, anm-plane or a plane having a slight off-angle from the a-plane or them-plane.

Although it is theoretically predicted that a semipolar plane provides alight-emitting device that exhibits excellent characteristics, alarge-diameter wafer is not available. Results of growth experiments,which is limited to the range of the off-angle described in PatentLiterature 2, are reported.

A crystal plane tilting from the c-plane at an angle larger than theoff-angle described in Patent Literature 1 has so called semi-polarity,in contrast to the c-plane, the a-plane and the m-plane. In order toutilize the semi-polarity in a gallium nitride-based semiconductor, itis desired that the tilt angle from the c-plane is larger than the angledescribed in Patent Literature 1.

In a growth of a gallium nitride-based semiconductor layer on asubstrate composed of a hexagonal compound, for example, a GaNsubstrate, strain is incorporated in the gallium nitride-basedsemiconductor layer due to a difference in crystal lattice constantbetween the hexagonal compound and the gallium nitride-basedsemiconductor layer. Relaxation of the strain in the galliumnitride-based semiconductor layer during the growth thereof formsdislocations. According to investigations conducted by the inventors, agallium nitride-based semiconductor device utilizing semipolar maysuppress the occurrence of the lattice relaxation and prevent theformation of dislocations in contrast to growth on a typical crystalplane, especially, the c-plane, the a-plane or the m-plane of ahexagonal compound.

It is an object of the present invention to provide a group III nitridesemiconductor device that can suppress the formation of dislocations dueto relaxation of strain incorporated in a gallium nitride-basedsemiconductor using semipolar, and it is another object of the presentinvention to provide an epitaxial wafer for the group III nitridesemiconductor device.

Solution to Problem

According to an aspect of the present invention, a group III nitridesemiconductor device comprises: (a) a supporting base having a primarysurface, the primary surface comprising a hexagonal compound, theprimary surface tilting by an off-angle of 10 degrees or more and lessthan 80 degrees with reference to a c-plane of the hexagonal compound;and (b) a semiconductor region provided on the primary surface of thesupporting base, the semiconductor region comprising a semiconductorlayer, the semiconductor layer comprising a hexagonal galliumnitride-based semiconductor different from the hexagonal compound. Atilt angle between a (0001) plane of the hexagonal compound of thesupporting base and a (0001) plane of the hexagonal galliumnitride-based semiconductor of the semiconductor layer is in a range of+0.05 degree to +2 degrees and −0.05 degree to −2 degrees, and thehexagonal gallium nitride-based semiconductor of the semiconductor layercomprises one of AlGaN and InGaN.

In the supporting base of the group III nitride semiconductor devicehaving the off-angle range described above, a tilt angle between the(0001) plane of the supporting base and the (0001) plane of thehexagonal gallium nitride-based semiconductor of 0.05 degree or more and2 degrees or less suppresses relaxation of strain in the hexagonalgallium nitride-based semiconductor to prevent an increase indislocation density in the hexagonal gallium nitride-basedsemiconductor.

In the group III nitride semiconductor device of the present invention,the <0001> direction of the supporting base is different from the <0001>direction of hexagonal gallium nitride-based semiconductor in atransmission electron microscopic image. In the group III nitridesemiconductor device, since the hexagonal gallium nitride-basedsemiconductor is elastically deformed in the plane that is parallel tothe primary surface of the supporting base, the occurrence of therelaxation of strain is suppressed, and the <0001> direction of GaN isdifferent from the <0001> direction of the hexagonal galliumnitride-based semiconductor.

The group III nitride semiconductor device of the present inventioncomprises: (a) a supporting base having a primary surface, the primarysurface comprising a hexagonal compound, the primary surface tilting byan off-angle of 10 degrees or more and less than 80 degrees withreference to a c-plane of a hexagonal compound; and (b) a semiconductorregion provided on the primary surface of the supporting base, thesemiconductor region comprising a semiconductor layer, and thesemiconductor layer comprising a hexagonal gallium nitride-basedsemiconductor different from the hexagonal compound. A <0001> directionof the hexagonal compound is indicated by a first axis in a transmissionelectron microscopic image, and a <0001> direction of the hexagonalgallium nitride-based semiconductor is indicated by a second axis in thetransmission electron microscopic image. The first axis extends in adirection different from a direction of the second axis in thetransmission electron microscopic image.

In the supporting base of the group III nitride semiconductor devicehaving the off-angle range described above, when the first axis thatindicates the <0001> direction of the supporting base extends in adirection different from the direction of the second axis that indicatesthe <0001> direction of the hexagonal gallium nitride-basedsemiconductor, this difference suppresses the relaxation of strain inthe hexagonal gallium nitride-based semiconductor and thus permits anincrease in the dislocation density of the hexagonal galliumnitride-based semiconductor.

The group III nitride semiconductor device of the present invention, asemiconductor stack may include an active layer composed of a hexagonalgallium nitride-based semiconductor, and the active layer may beprovided so as to emit light having an emission peak in the wavelengthrange of 400 to 550 nm. The active layer may include an InGaN welllayer, and the group III nitride semiconductor device may be alight-emitting diode or a semiconductor laser. The group III nitridesemiconductor device has satisfactory emission performance due tosuppression of an increase in the dislocation density.

The supporting base of the group III nitride semiconductor deviceaccording to the present invention may be composed of sapphire, SiC orGaN. In the group III nitride semiconductor device, the supporting basecomposed of such a material can use the effect of the suppression of therelaxation of strain.

The supporting base of the group III nitride semiconductor device of thepresent invention may be composed of the gallium nitride-basedsemiconductor. The supporting base has a semipolar primary surface.Since the supporting base and the semiconductor layer of the group IIInitride semiconductor device are composed of gallium nitride-basedsemiconductor, the gallium nitride-based semiconductor with high crystalquality can be grown on the supporting base of the gallium nitride-basedsemiconductor.

The supporting base of the group III nitride semiconductor deviceaccording to the present invention may have a gallium nitride-basedsemiconductor region with a threading dislocation density of 1×10⁷ cm⁻²or less. In the group III nitride semiconductor device, the galliumnitride-based semiconductor can be grown on the gallium nitride-basedsemiconductor region with low dislocations. Thus, the occurrence of therelaxation of strain is made reduced because the dislocations comingfrom the substrate are low.

In the group III nitride semiconductor device according to the presentinvention, the hexagonal compound of the supporting base is GaN, and thetilt angle is defined by an angle between the (0001) plane of the GaN ofthe supporting base and the (0001) plane of the hexagonal galliumnitride-based semiconductor of the semiconductor layer.

In the group III nitride semiconductor device, using a high-quality GaNwafer can reduce the occurrence of the strain relaxation caused bydislocations from the substrate.

In the group III nitride semiconductor device according to the presentinvention, the hexagonal gallium nitride-based semiconductor of thesemiconductor layer may be composed of InGaN. The tilt angle is definedby an angle between the (0001) plane of GaN of the supporting base andthe (0001) plane of InGaN of the semiconductor layer. The group IIInitride semiconductor device can reduce the occurrence of relaxation ofstrain in the InGaN layer. Alternatively, in the group III nitridesemiconductor device according to the present invention, the hexagonalgallium nitride-based semiconductor of the semiconductor layer may becomposed of AlGaN. The tilt angle is defined by an angle between the(0001) plane of GaN of the supporting base and the (0001) plane of AlGaNof the semiconductor layer. The group III nitride semiconductor devicecan reduce the occurrence of relaxation of strain in the AlGaN layer.

The hexagonal gallium nitride-based semiconductor of the group IIInitride semiconductor device according to the present invention iselastically deformed in the plane that is parallel to the primarysurface of the supporting base. In the group III nitride semiconductordevice, the relaxation of strain is reduced because the hexagonalgallium nitride-based semiconductor is elastically deformed in the planethat is parallel to the primary surface of the supporting base.

The supporting base of the group III nitride semiconductor deviceaccording to the present invention may include, for example, a sapphiresubstrate or an SiC substrate. For example, the supporting base mayinclude an A-plane sapphire substrate and a GaN layer grown on thesapphire substrate.

An epitaxial wafer according to another aspect of the present inventioncomprises: (a) a wafer having a primary surface, the primary surfacecomprising a hexagonal compound, the primary surface tilting by anoff-angle of 10 degrees or more and less than 80 degrees with referenceto a c-plane of the hexagonal compound; and (b) a semiconductor regionprovided on the primary surface of the wafer, the semiconductor regioncomprising a semiconductor layer, the semiconductor layer comprising ahexagonal gallium nitride-based semiconductor different from thehexagonal compound. A tilt angle between a (0001) plane of the hexagonalcompound of the wafer and a (0001) plane of the hexagonal galliumnitride-based semiconductor of the semiconductor layer is in a range of+0.05 degree to +2 degrees and −0.05 degree to −2 degrees, and thehexagonal gallium nitride-based semiconductor of the semiconductor layercomprises one of AlGaN and InGaN.

In the wafer of the epitaxial wafer having the off-angle range describedabove, a tilt angle (absolute value) between the (0001) plane of thesupporting base and the (0001) plane of the hexagonal galliumnitride-based semiconductor of 0.05 degree or more and 2 degrees or lesscan suppress the relaxation of strain in the hexagonal galliumnitride-based semiconductor and thus prevents an increase in thedislocation density of the hexagonal gallium nitride-basedsemiconductor.

In the epitaxial wafer of the present invention, the <0001> direction ofthe hexagonal compound of the wafer is different from the <0001>direction of hexagonal gallium nitride-based semiconductor in atransmission electron microscopic image. In the epitaxial wafer, sincethe hexagonal gallium nitride-based semiconductor is elasticallydeformed in the plane that is parallel to the primary surface of thewafer, the occurrence of the strain relaxation is suppressed, so thatthe <0001> direction of the hexagonal compound is different from the<0001> direction of the hexagonal gallium nitride-based semiconductor.

The epitaxial wafer of the present invention comprises: (a) a waferhaving a primary surface, the primary surface comprising a hexagonalcompound, the primary surface tilting by an off-angle of 10 degrees ormore and less than 80 degrees with reference to a c-plane of a hexagonalcompound; and (b) a semiconductor region provided on the primary surfaceof the wafer, the semiconductor layer comprising a hexagonal galliumnitride-based semiconductor different from the hexagonal compound, andthe semiconductor region comprising a semiconductor layer. A <0001>direction of the hexagonal compound is indicated by a first axis in atransmission electron microscopic image, and a <0001> direction of thehexagonal gallium nitride-based semiconductor is indicated by a secondaxis in the transmission electron microscopic image. The first axisextends in a direction different from a direction of a second axis inthe transmission electron microscopic image.

In the wafer of the epitaxial wafer having the off-angle in theabove-described range, since the <0001> direction of the hexagonalcompound is different from the direction of the <0001> direction of thehexagonal gallium nitride-based semiconductor, this difference cansuppress the strain relaxation in the hexagonal gallium nitride-basedsemiconductor and thus prevents an increase in the dislocation densityof the hexagonal gallium nitride-based semiconductor.

In the epitaxial wafer of the present invention, the semiconductorregion may include an active layer composed of the hexagonal galliumnitride-based semiconductor, and the peak wavelength of aphotoluminescence spectrum of the active layer may be in between 400 nmand 550 nm. In the present invention, suppression of an increase indislocation density provides an epitaxial wafer for the group IIInitride semiconductor device with excellent emission performance.

In the epitaxial wafer of the present invention, the wafer may becomposed of sapphire, SiC or GaN. In the epitaxial wafer, the wafercomposed of such a material can use the effect of the suppression of thestrain relaxation.

In the epitaxial wafer according to the present invention, the wafer maybe composed of a gallium nitride-based semiconductor, and the wafer hasa semipolar primary surface. Since the supporting base and thesemiconductor layer of the epitaxial wafer are composed of a galliumnitride-based semiconductor, the gallium nitride-based semiconductorwith high crystal quality can be grown on the supporting base of thegallium nitride-based semiconductor.

The epitaxial wafer according to the present invention may have agallium nitride-based semiconductor region with a threading dislocationdensity of 1×10⁷ cm⁻² or less. In the epitaxial wafer, the galliumnitride-based semiconductor can be grown on the gallium nitride-basedsemiconductor region with low dislocations. Thus, lowering dislocationscoming from the wafer reduces the occurrence of the strain relaxation.

In the epitaxial wafer of the present invention, the hexagonal compoundof the wafer can be GaN, and the tilt angle is defined by an anglebetween the (0001) plane of the GaN of the wafer and the (0001) plane ofthe hexagonal gallium nitride-based semiconductor of the semiconductorlayer.

Using a high-quality GaN wafer for forming the epitaxial wafer reducesthe occurrence of the strain relaxation caused by dislocations comingfrom the wafer.

In the epitaxial wafer according to the present invention, the hexagonalgallium nitride-based semiconductor of the semiconductor layer may becomposed of InGaN. The tilt angle may be defined by an angle between the(0001) plane of GaN of the wafer and the (0001) plane of InGaN of thesemiconductor layer. This epitaxial wafer can reduce the occurrence ofthe strain relaxation in the InGaN layer. Alternatively, in theepitaxial wafer according to the present invention, the hexagonalgallium nitride-based semiconductor of the semiconductor layer may becomposed of AlGaN, and the tilt angle is defined by an angle between the(0001) plane of GaN of the wafer and the (0001) plane of AlGaN of thesemiconductor layer. This epitaxial wafer can reduce the occurrence ofthe strain relaxation in the AlGaN layer.

The hexagonal gallium nitride-based semiconductor of the epitaxial waferof the present invention is elastically deformed in the plane that isparallel to the primary surface of the wafer. This epitaxial wafer canreduce the strain relaxation because of the elastic deformation of thehexagonal gallium nitride-based semiconductor caused in the planeparallel to the primary surface of the supporting base.

The foregoing and other objects, features, and advantages of the presentinvention will become more readily apparent from the following detaileddescription of a preferred embodiment of the invention, which proceedswith reference to the accompanying drawings.

Advantageous Effects of Invention

As described above, one aspect of the present invention provides a groupIII nitride semiconductor device that can suppress the formation ofdislocations due to relaxation of strain incorporated in a galliumnitride-based semiconductor using semi-polarity. Another aspect of thepresent invention provides an epitaxial wafer for the group III nitridesemiconductor device.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view including schematic diagrams that show the relationshipbetween the lattice constant of a hexagonal compound and the latticeconstant of a hexagonal gallium nitride-based semiconductor.

FIG. 2 is a view including schematic diagrams each of which illustratesa semiconductor layer of a hexagonal gallium nitride-based semiconductorgrown on a supporting base of a hexagonal compound.

FIG. 3 is a view including schematic diagrams each of which shows therelationship between the lattice constant of a hexagonal compound andthe lattice constant of a hexagonal gallium nitride-based semiconductor.

FIG. 4 is a view including schematic diagrams each of which illustratesa semiconductor layer of a hexagonal gallium nitride-based semiconductorgrown on a supporting base of a hexagonal compound.

FIG. 5 is a schematic view illustrating a structure of a light-emittingdiode according to an embodiment of the present invention.

FIG. 6 is a schematic view illustrating a structure of a semiconductorlaser according to an embodiment of the present invention.

FIG. 7 is a flowchart of primary steps in a method of fabricatingprocess a light-emitting diode.

FIG. 8 is a flowchart of primary steps in the method of fabricating thelight-emitting diode.

FIG. 9 is a graph showing typical PL spectra.

FIG. 10 is a graph showing an observed reciprocal lattice mapping image.

FIG. 11 is a magnified view showing an image of an active layer and itsvicinity in the LED structure shown in FIG. 5.

FIG. 12 is a schematic view illustrating a structure of an epitaxialwafer E26 and an image, associated with the structure, on whichmeasuring points is depicted.

FIG. 13 is a view including images that show diffraction patternsobserved at the three measuring points.

FIG. 14 is a view including electron diffraction patterns of anepitaxial wafer.

FIG. 15 is a view including magnified images of the electron diffractionpatterns of the epitaxial wafer.

FIG. 16 is a view including graphs that show calculated values of tiltangles for InGaN and AlGaN, respectively.

DESCRIPTION OF EMBODIMENTS

The teaching of the present invention can readily be understood from thefollowing detailed description with reference to the accompanyingdrawings by way of examples. Embodiments according to a group IIInitride semiconductor device and an epitaxial wafer of the presentinvention will be described with reference to the accompanying drawings.The same components are designated by the same reference numerals,wherever possible.

A number of investigations have been made on crystal growth on thea-plane, the m-plane, and a crystal plane having a slight off-angle fromthe a-plane or the m-plane. However, what is desired is to provide theteaching of relaxation of strain incorporated in a hexagonal galliumnitride-based semiconductor for a semiconductor device that uses asemipolar gallium nitride-based semiconductor. The hexagonal galliumnitride-based semiconductor is grown on the primary surface of asubstrate composed of a hexagonal compound, and the primary surface ofthe substrate tilts by an off-angle of 10 degrees or more to less than80 degrees with reference to the c-plane of the hexagonal compound.

The group III nitride semiconductor device according to the embodimentincludes a supporting base having such an off-angle, and a semiconductorregion provided on the primary surface of the supporting base. Thesemiconductor region may be composed of a semiconductor stack includinga semiconductor layer composed of a hexagonal gallium nitride-basedsemiconductor. An epitaxial wafer for the group III nitridesemiconductor device according to the embodiment also includes a waferhaving the off-angle, and a semiconductor region provided on the primarysurface of the wafer. The semiconductor region includes a semiconductorfilm composed of a hexagonal gallium nitride-based semiconductor.

If a hexagonal compound is different from hexagonal galliumnitride-based semiconductor of the semiconductor layer, the latticeconstant (for example, the lattice constant in the c-axis) of thehexagonal compound also is different from the lattice constant (forexample, the lattice constant in the c-axis) of the hexagonal galliumnitride-based semiconductor. In this instance, the hexagonal galliumnitride-based semiconductor incorporates strain caused by the differencein the lattice constant. Strains are relaxed in crystal to formstructural defects (for example, dislocations). However, an increase inthe dislocations degrades the crystal quality of hexagonal galliumnitride-based semiconductor. Thus, a reduction in the dislocations isdesired.

FIG. 1 is a view including schematic diagrams that show therelationships between the lattice constant of hexagonal compounds andthe lattice constants of hexagonal gallium nitride-based semiconductors.In the materials shown in Parts (a) to (c) of FIG. 1, the latticeconstants of the hexagonal compounds are smaller than those of thehexagonal gallium nitride-based semiconductors. The vectors C shown inthe schematic diagrams of FIG. 1 indicate the respective directions ofthe c-axis in the hexagonal compounds. The reference symbol SSUB_(C)indicates the c-plane of the hexagonal compound. The reference symbolSLAY_(C1) indicates the c-plane of the hexagonal gallium nitride-basedsemiconductor. The reference symbols 11 a, 11 b and 11 c indicate waferscomposed of the hexagonal compound, while the reference characters 13 a,13 b and 13 c indicate semiconductor layers composed of the hexagonalgallium nitride-based semiconductor. The maximum of the distance betweentwo points on the edges of the wafers 11 a, 11 b, and 11 c may be 45 mmor more. The area of the primary surface of the wafers 11 a, 11 b, and11 c can be, for example, 15 cm² or more.

With reference to Part (a) of FIG. 1, the wafer 11 a is provided, andhas a primary surface of polar c-plane. The hexagonal galliumnitride-based semiconductor 13 a is to be grown on the wafer 11 a. Theoriginal lattice constant DLAY_(P1) inherent to the hexagonal galliumnitride-based semiconductor 13 a is larger than the original latticeconstant DSUB_(P) inherent to the hexagonal compound of the wafer 11 a.This lattice constant is defined in the direction of the a-axis or them-axis.

With reference to Part (b) of FIG. 1, the wafer 11 b is prepared whichhas a primary surface of a nonpolar a-plane (or m-plane). The hexagonalgallium nitride-based semiconductor 13 b is to be grown on the wafer 11b. The lattice constant DLAY_(N1) of the hexagonal gallium nitride-basedsemiconductor 13 b is larger than the lattice constant DSUB_(N) of thehexagonal compound of the wafer 11 b. This lattice constant is definedin the direction of the c-axis.

With reference to Part (c) of FIG. 1, the wafer 11 c is prepared whichhas a semipolar primary surface. The primary surface of the wafer 11 cis composed of the hexagonal compound, and the primary surface tilts byan off-angle of 10 degrees or more and less than 80 degrees withreference to the c-plane of the hexagonal compound. The hexagonalgallium nitride-based semiconductor 13 c is to be grown on the wafer 11c. In the wafer 11 c having the tilting primary surface, the latticeconstants as shown in Parts (a) and (b) of FIG. 1 are represented inassociation with the primitive lattice in terms of the axial directionof the primitive lattice. However, the relationship between the size ofthe primitive lattice DLAY_(H1) of the hexagonal gallium nitride-basedsemiconductor 13 c and the size of the primitive lattice DSUB_(H) of thehexagonal compound of the wafer 11 c is the same as those shown in Parts(a) and (b) of FIG. 1, and thus the relationship DLAY_(H1)>DSUB_(H) issatisfied.

FIG. 2 is a view including schematic diagrams, each of which illustratesa semiconductor layer of a hexagonal gallium nitride-based semiconductorgrown on a supporting base of a hexagonal compound. In the materialsshown in FIG. 1, the lattice constant of the hexagonal compound issmaller than that of the hexagonal gallium nitride-based semiconductor.The vectors C shown in Parts (a), (b) and (c) of FIG. 2 indicate thedirection of the c-axis in the hexagonal compound and the hexagonalgallium nitride-based semiconductor. If the hexagonal galliumnitride-based semiconductors are grown on the wafers 11 a, 11 b and 11c, the original lattice constants inherent to the hexagonal galliumnitride-based semiconductors 13 a, 13 b and 13 c vary depending on thelattice constants of the wafer 11 a, 11 b and 11 c, to form hexagonalgallium nitride-based semiconductors 15 a, 15 b, and 15 c, respectively.Consequently, the hexagonal gallium nitride-based semiconductors 15 a,15 b, and 15 c involve strains. If lattice defects, for example,dislocations are induced during the crystal growth, they release strainsto degrade the crystal quality. Thus, the occurrence of the strainrelaxation is not desirable.

With reference to Part (c) of FIG. 2, the semipolar hexagonal galliumnitride-based semiconductor 15 c also incorporates strain. In contrastto the hexagonal gallium nitride-based semiconductors 15 a and 15 bgrown on the c-plane, the a-plane and the m-plane, in the hexagonalgallium nitride-based semiconductor 15 c in which the strain remainswithout relaxation, the crystal plane such as the c-plane SSUB_(C) ofthe wafer 11 c extends parallel to the reference plane S_(R1) that isnot parallel to the corresponding crystal plane, for example, thec-plane SLAY_(C1) of the hexagonal gallium nitride-based semiconductor15 c. The angular difference between the above two c-planes is estimatedfrom results of the X-ray diffractometry, as described below.

Where dislocations are formed during the growth of the hexagonal galliumnitride-based semiconductor onto the wafer 11 c to relax strainstherein, the number of dislocations that are created in the hexagonalgallium nitride-based semiconductor grown on the wafer 11 c is verylarge. In such a case, the strain relaxation results in the deformationof the hexagonal gallium nitride-based semiconductor in which thecrystal plane, for example, the c-plane SLAY_(C1) of the hexagonalgallium nitride-based semiconductor extends parallel to a predefinedcrystal plane, for example, the c-plane SSUB_(C) of the wafer 11 b. Forexample, when the hexagonal compound is GaN, the hexagonal galliumnitride-based semiconductor is composed of, for example, InGaN orInAlGaN. The lattice constant of InGaN depends on its indium compositionand is larger than that of GaN.

Where the hexagonal gallium nitride-based semiconductor 15 c alsoincorporates strain, the occurrence of the relaxation in its growthbecomes very low. In such a case, the result of the X-ray diffractometrydemonstrates that the tilt angle a (absolute value) between the c-planeSLAY_(C1) and the reference plane S_(R1) in the hexagonal galliumnitride-based semiconductor 15 c is 0.05 degree or more. In addition,the tilt angle α (absolute value) is 2 degree or less. For example, whenthe hexagonal gallium nitride-based semiconductor is composed of InGaNand the wafer 11 c is composed of GaN, the tilt angle is defined by anangle between the (0001) plane of GaN of the wafer 11 c and the (0001)plane of InGaN of the semiconductor layer, for example. The occurrenceof strain relaxation in the InGaN layer is reduced, and the number ofdislocations formed during the growth of the hexagonal galliumnitride-based semiconductor 15 c is very small. The above relationshipon the tilt angle holds over the entire primary surface of the wafer 11c. In order to satisfy the relationship of the tilt angle, the indiumcomposition is preferably 0.07 or more and 0.35 or less.

Although the above explanation is made, as an example, on a singlesemiconductor layer composed of the hexagonal gallium nitride-basedsemiconductor grown on the wafer 11 c with reference to FIGS. 1 and 2,the incorporation of strain is also applicable to a structure thatincludes a semiconductor stack including a plurality of semiconductorlayers grown on the wafer 11 c.

As can be seen from the above description, in the wafer 11 c having theoff-angle range described above, when a tilt angle (absolute value)between the (0001) plane of the wafer 11 c and the (0001) plane of thehexagonal gallium nitride-based semiconductor 15 c is in the range of0.05 degree or more and 2 degrees or less, the strain relaxation issuppressed in the hexagonal gallium nitride-based semiconductor 15 c toavoid an increase in the dislocation density of the hexagonal galliumnitride-based semiconductor 15 c in the group III nitride semiconductordevice.

With reference to Part (c) of FIG. 2, the axis A_(R) indicates the<0001> direction of the hexagonal gallium nitride-based semiconductor 15c. In a transmission electron beam diffraction image, the <0001>direction of the wafer 11 c is different from the axis A_(R). In orderto clearly show the angular difference, an auxiliary axis A_(R) is alsodepicted in conjunction with the c-axis vector C of the wafer 11 c. Theangle β between the auxiliary axis A_(R) and the vector C is related tostrain incorporated therein, and the angle β is substantially equal tothe angle α. When these angles (absolute values) are 0.05 degree or moreand 2 degrees or less, the relaxation of strain is suppressed in thehexagonal gallium nitride-based semiconductor 15 c, and thus an increasein the density of dislocations in the hexagonal gallium nitride-basedsemiconductor 15 c in the group III nitride semiconductor device isavoided. The hexagonal gallium nitride-based semiconductor 15 c iselastically deformed in the plane parallel to the primary surface of thewafer 11 c to suppress the occurrence of strain relaxation, so that the<0001> direction of the wafer 11 c is different from the <0001>direction of the hexagonal gallium nitride-based semiconductor 15 c. Theabove relationships of the tilt angle is satisfied over the entireprimary surface of the wafer 11 c.

FIG. 3 includes other schematic views showing the relationship betweenthe lattice constant of a hexagonal compound and the lattice constant ofa hexagonal gallium nitride-based semiconductor. In the materials shownin FIG. 3, the lattice constant of the hexagonal compound is larger thanthat of the hexagonal gallium nitride-based semiconductor. The vectors Cshown in Parts (a), (b) and (c) in FIG. 3 indicate the direction of thec-axis of the hexagonal compound, and the reference symbols SSUB_(P) andSLAY_(P) represent the c-planes of the hexagonal compound and thehexagonal gallium nitride-based semiconductor, respectively. Thereference symbols 11 a, 11 b and 11 c indicate supporting bases composedof the hexagonal compound, and the reference symbols 17 a, 17 b and 17 cindicate semiconductor layers composed of the hexagonal galliumnitride-based semiconductor.

With reference to Part (a) of FIG. 3, the wafer 11 a is prepared whichhas a primary surface of polar c-plane. The hexagonal galliumnitride-based semiconductor 17 a is to be grown on the wafer 11 a. Theoriginal lattice constant DLAY_(P2) inherent to the hexagonal galliumnitride-based semiconductor 17 a is smaller than the original latticeconstant DSUB_(P) inherent to the hexagonal compound of the wafer 11 a.This lattice constant is defined in the direction of the a-axis or them-axis.

With reference to Part (b) of FIG. 3, the wafer 11 b is provided, andhas a primary surface of a nonpolar a-plane (or m-plane). The hexagonalgallium nitride-based semiconductor 17 b is to be grown on the wafer 11b. The original lattice constant DLAY_(N2) inherent to the hexagonalgallium nitride-based semiconductor 17 b is smaller than the originallattice constant DSUB_(N) inherent to the hexagonal compound of thewafer 11 b. This lattice constant is defined in the direction of thec-axis.

With reference to Part (c) of FIG. 3, the wafer 11 c is prepared whichhas a semipolar primary surface. The primary surface of the wafer 11 cis composed of the hexagonal compound, and the primary surface tilts byan off-angle of 10 degrees or more and less than 80 degrees withreference to the c-plane of the hexagonal compound. The hexagonalgallium nitride-based semiconductor 17 c is to be grown on the wafer 11c. In the wafer 11 c having the tilting primary surface, the latticeconstant as shown in Parts (a) and (b) of FIG. 3 cannot be representedin association with the primitive lattice in terms of the axialdirection of the primitive lattice. However, the relationship betweenthe size of the primitive lattice DLAY_(H2) of the hexagonal galliumnitride-based semiconductor 17 c and the size of the primitive latticeDSUB_(H) of the hexagonal compound of the wafer 11 c is the same asthose shown in Parts (a) and (b) of FIG. 3.

FIG. 4 includes schematic views illustrating a semiconductor layer of ahexagonal gallium nitride-based semiconductor grown on a supporting baseof a hexagonal compound. In the material shown in FIG. 3, the latticeconstant of the hexagonal compound is smaller than that of the hexagonalgallium nitride-based semiconductor. The vectors C shown in Parts (a),(b) and (c) of FIG. 4 indicate the direction of the c-axis of thehexagonal compound and hexagonal gallium nitride-based semiconductor.The hexagonal gallium nitride-based semiconductors are grown on thewafers 11 a, 11 b and 11 c to form hexagonal gallium nitride-basedsemiconductors 19 a, 19 b and 19 c, respectively. The original latticeconstants inherent to the hexagonal gallium nitride-based semiconductors17 a, 17 b and 17 c vary depending on the lattice constants of the wafer11 a, 11 b and 11 c. Consequently, the hexagonal gallium nitride-basedsemiconductors 19 a, 19 b and 19 c incorporate strains. If latticedefects, for example, dislocations are formed during a crystal growth,they cause the relaxation of strains therein to degrade the crystalquality. Accordingly, the occurrence of the strain relaxation is notdesirable.

With reference to Part (c) of FIG. 4, the semipolar hexagonal galliumnitride-based semiconductor 19 c also incorporates strains. In contrastto the hexagonal gallium nitride-based semiconductors 19 a and 19 bgrown on the c-plane, the a-plane and the m-plane, in the hexagonalgallium nitride-based semiconductor 19 c in which the strain remainswithout relaxation, the crystal plane such as the c-plane SSUB_(C) ofthe wafer 11 c extends parallel to the reference plane S_(R2) that isnot parallel to the corresponding crystal plane, for example, thec-plane SLAY_(C1) of the hexagonal gallium nitride-based semiconductor19 c. This angular difference is demonstrated by the measurements of theX-ray diffractometry, as described below.

Where dislocations are formed during the growth of the hexagonal galliumnitride-based semiconductor onto the wafer 11 c to relax strainstherein, that is, a significantly large number of dislocations areformed, the hexagonal gallium nitride-based semiconductor is deformed asa result of the strain relaxation such that the crystal plane, forexample, the c-plane SLAY_(C1) of the hexagonal gallium nitride-basedsemiconductor extends parallel to the crystal plane, for example, thec-plane SSUB_(C) of the wafer 11 b. For example, when the hexagonalcompound is GaN, the hexagonal gallium nitride-based semiconductor iscomposed of, for example, AlGaN or InAlGaN. The lattice constant ofAlGaN depends on its aluminum composition, and is smaller than that ofGaN.

Where the hexagonal gallium nitride-based semiconductor 19 c alsoincorporates strain, that is, the occurrence of the dislocations andcracks in its growth becomes very low, the results of the X-raydiffractometry demonstrates that the tilt angle γ (absolute value)between the c-plane SLAY_(C1) and the reference plane S_(R2) in thehexagonal gallium nitride-based semiconductor 19 c is 0.05 degree ormore. In addition, the tilt angle γ (absolute value) is 2 degree orless. The above relationship on the tilt angle is satisfied over theentire primary surface of the wafer 11 c.

The above explanation is made on a single semiconductor layer composedof the hexagonal gallium nitride-based semiconductor grown on the wafer11 c, as an example, with reference to FIGS. 3 and 4, and theincorporation of strain is also applicable to a structure that has asemiconductor stack including a plurality of semiconductor layers grownon the wafer 11 c.

As can be seen from the above description, in the wafer 11 c having theoff-angle range described above, when a tilt angle between the (0001)plane of the wafer 11 c and the (0001) plane of the hexagonal galliumnitride-based semiconductor 19 c is in a range of 0.05 degree or moreand 2 degrees or less, the strain relaxation is suppressed in thehexagonal gallium nitride-based semiconductor 19 c to reduce theformation of dislocations in the hexagonal gallium nitride-basedsemiconductor 19 c of the group III nitride semiconductor device. Whenthe hexagonal gallium nitride-based semiconductor and the wafer 11 c arecomposed of, for example, AlGaN and GaN, respectively, the tilt angle isdefined by an angle between the (0001) plane of GaN of the wafer 11 cand the (0001) plane of AlGaN of the semiconductor layer of thehexagonal gallium nitride-based semiconductor, and the occurrence of thestrain relaxation is reduced in the AlGaN layer. In order to satisfy therelationship of the tilt angle, the aluminum composition is preferably0.2 or less.

With reference to Part (c) of FIG. 4, the axis A_(R) indicates the<0001> direction of the hexagonal gallium nitride-based semiconductor 19c. The <0001> direction of the wafer 11 c is different from the axisA_(R) in a transmission electron beam diffraction image. In order toclearly show the angular difference, an auxiliary axis A_(R) is depictedin conjunction with the c-axis vector C of the wafer 11 c. The angle Sbetween the auxiliary axis A_(R) and the vector C relates to theincorporation of strain, and the angle δ is substantially equal to theangle γ. When these angles are 0.05 degree or more and 2 degrees orless, the relaxation of strain is suppressed in the hexagonal galliumnitride-based semiconductor 19 c, to prevent the dislocation density inthe hexagonal gallium nitride-based semiconductor 19 c from increasingin the group III nitride semiconductor device. Since the hexagonalgallium nitride-based semiconductor 19 c is elastically deformed in theplane parallel to the primary surface of the wafer 11 c, the occurrenceof relaxation of strain is suppressed and the <0001> direction of thewafer 11 c is different from the <0001> direction of the hexagonalgallium nitride-based semiconductor 19 c. The tilt angle relationship asabove is satisfied over the entire primary surface of the wafer 11 c.

The hexagonal gallium nitride-based semiconductor for the semiconductorstack may be composed of, for example, InAlGaN, in addition to InGaN orAlGaN. The tilt angle relationship as above holds over the entireprimary surface of the wafer 11 c.

When the semiconductor stack in each of the epitaxial wafers E1 and E2includes an active layer composed of a hexagonal gallium nitride-basedsemiconductor, the group III nitride semiconductor device may be asemiconductor light-emitting device such as a light-emitting diode or asemiconductor laser. The peak wavelength of a photoluminescence spectrumof the active layer may be in the range of 400 nm or more to 550 nm orless. The group III nitride semiconductor light-emitting devicesfabricated from these epitaxial wafers E1 and E2 have excellent emissionproperties because an increase in the dislocation density is suppresseddue to elastic deformation of the semiconductor layers therein.

After other components, such as electrodes, are fabricated on theepitaxial wafers for group III nitride semiconductor light-emittingdevices to form wafer products, these wafer products are separated intoa number of semiconductor light-emitting devices. The semiconductorlight-emitting device includes an active layer in a semiconductor stackon a supporting base formed by separation of the wafer 11 c. In thesemiconductor light-emitting device such as a light-emitting diode or asemiconductor laser, the active layer is provided so as to haveelectroluminescence with an emission peak in the wavelength range from400 to 550 nm. The group III nitride semiconductor light-emitting devicehas satisfactory emission properties due to suppression of an increasein the dislocation density. In an embodiment, the active layer may havea quantum well structure including a well layer and a barrier layer. Theactive layer includes, for example, an InGaN well layer. The indiumcomposition of the well layer preferably ranges from 0.07 to 0.35 foremission in the wavelength range from 400 to 550 nm. The thickness ofthe well layer preferably ranges from 1.5 to 10 nm.

The wafer 11 c may be composed of, for example, sapphire or SiC. In thegroup III nitride semiconductor device, the supporting base composed ofsuch a material can utilize the suppression of the relaxation of strain,as described above. For example, the supporting base may include anA-plane sapphire substrate and a GaN layer grown on the sapphiresubstrate. The A-plane sapphire substrate allows the epitaxial growth ofGaN having a primary surface of the (10-12) plane. The epitaxial film ofGaN having a dislocation density of 1×10⁺⁸ cm⁻² or less can provide theadvantage in a portion between an n-type GaN underlying layer and anInGaN layer of the present embodiment.

The wafer 11 c may be composed of a gallium nitride-based semiconductor.The wafer 11 c has a semipolar primary surface. In this group IIInitride semiconductor device, since the wafer 11 c and the semiconductorlayers 15 c and 19 c are composed of gallium nitride-basedsemiconductors, the gallium nitride-based semiconductor having a highcrystal quality can be grown on the gallium nitride-based semiconductorsupporting base. The gallium nitride-based semiconductor supporting basepreferably includes a gallium nitride-based semiconductor region havinga threading dislocation density of 1×10⁷ cm⁻² or less. The threadingdislocation density may be defined, for example, in the c-plane of thegallium nitride-based semiconductor supporting base. The galliumnitride-based semiconductor can be grown on this gallium nitride-basedsemiconductor region with a low dislocation density. Thus, theoccurrence of the strain relaxation is reduced because the density ofdislocations coming from the wafer is low.

In the wafer 11 c composed of GaN, the tilt angles α and γ are definedby the (0001) plane of GaN of the supporting base 11 c and the (0001)planes of the hexagonal gallium nitride-based semiconductors of thesemiconductor layers 15 c and 19 c, respectively. The GaN wafer withhigh quality and a large diameter can reduce the occurrence of therelaxation of strain due to dislocations from the GaN wafer. In the GaNwafer having a semipolar primary surface, the off-angle varies over theentire primary surface. The requirements on the tilt angles α and γ holdregardless of the above distribution of the off-angle.

FIG. 5 is a schematic view illustrating a structure of a light-emittingdiode according to an embodiment of the present invention. Thelight-emitting diode 21 a includes a supporting base 23 and a stack ofsemiconductor layers 25 a provided on a primary surface 23 a of thesupporting base 23. The primary surface 23 a of the supporting base 23tilts by an off-angle of 10 degrees or more and less than 80 degreeswith reference to the c-plane. The supporting base 23 may be composed ofa single crystal. The semiconductor stack 25 a includes an active layer27 having an emission peak in a wavelength range from 400 to 550 nm.

The active layer 27 is provided between an n-type gallium nitride-basedsemiconductor region 29 and a p-type gallium nitride-based semiconductorregion 31. The n-type gallium nitride-based semiconductor region 29includes a buffer layer 33 a and an n-type GaN layer 33 b. The p-typegallium nitride-based semiconductor region 31 includes anelectron-blocking layer 35 a and a contact layer 35 b. The active layer27 has a multiple quantum well structure including well layers 37 a andbarrier layers 37 b that are alternately arranged. A first electrode 39a such as an anode is provided on the contact layer 35 b, and a secondelectrode 39 b such as a cathode is provided on the backside surface 23b of the supporting base 23.

An example of the structure of a light-emitting diode is as follows:

-   Supporting base 23: n-type GaN substrate;-   Buffer layer 33 a: Si-doped n-type Al_(0.06)Ga_(0.94)N layer, 50 nm;-   N-type GaN layer 33 b: Si-doped n-type GaN layer, 2 μm;-   Well layer 37 a: three undoped In_(0.18)Ga_(0.82)N layers, 5 nm;-   Barrier layer 37 b: undoped GaN layer, 13 nm;-   Electron-blocking layer 35 a: Mg-doped p-type Al_(0.08)Ga_(0.92)N    layer, 20 nm;-   Contact layer 35 b: Mg-doped p-type GaN layer, 50 nm.

In FIG. 5, the direction of the c-axis of the GaN supporting base isindicated by an axis A_(R3). The tilt angle “A,” which is formed betweenthe (0001) plane (the reference plane S_(R3) shown in FIG. 5) of the GaNsupporting base and the (0001) plane of the AlGaN buffer layer 33 a, is0.05 degree or more and 2 degrees or less. The tilt angle B, which isformed between the (0001) plane of the GaN supporting base (thereference plane S_(R4) shown in FIG. 5) and the (0001) plane of the welllayer 37 a, is 0.05 degree or more and 2 degrees or less. The tiltangles “A” and “B” are formed in a direction opposite to each other withreference to the c-plane of the GaN supporting base. The tilt anglerelations hold over the entire primary surface of the supporting base23.

FIG. 6 is a schematic view illustrating a structure of a semiconductorlaser according to an embodiment of the present invention. Thesemiconductor laser 21 b includes a supporting base 23 and a stack ofsemiconductor layers 25 b provided on a primary surface 23 a of thesupporting base 23. The supporting base 23 may have a single crystalregion onto which the light emitting region of the active layer isaligned. The semiconductor stack 25 b includes an n-type galliumnitride-based semiconductor region 41, an optical waveguide region 45,and a p-type gallium nitride-based semiconductor region 51. The opticalwaveguide region 45 is provided between the n-type gallium nitride-basedsemiconductor region 41 and the p-type gallium nitride-basedsemiconductor region 51. The optical waveguide region 45 includes anactive layer 47 a, and the active layer 47 a is provided between opticalguiding layers 47 b and 47 c. The active layer 47 a has a lasingwavelength within the range of 400 to 550 nm. The active layer 47 a hasa multiple quantum well structure including well layers 49 a and barrierlayers 49 b that are alternately arranged. The n-type galliumnitride-based semiconductor region 41 includes an n-type cladding layer43. The p-type gallium nitride-based semiconductor region 51 includes anelectron-blocking layer 53 a, a p-type cladding layer 53 b and a p-typecontact layer 53 c. A first electrode 55 a such as an anode is providedon the contact layer 53 c, and a second electrode 55 b such as a cathodeis provided on the backside surface 23 b of the supporting base 23.

An example of the structure of a laser diode (LD) is as follows:

-   Supporting base 23: n-type GaN substrate;-   N-type cladding layer 43: Si-doped n-type Al_(0.03)Ga_(0.97)N layer,    2 μm;-   N-side light-guiding layer 47 b: undoped In_(0.02)Ga_(0.98)N layer,    100 nm;-   Well layer 49 a: three undoped In_(0.08)Ga_(0.92)N layers, 5 nm;-   Barrier layer 49 b: undoped GaN layer, 15 nm;-   P-side light-guiding layer 47 c: undoped In_(0.02)Ga_(0.98)N layer,    100 nm;-   Electron-blocking layer 53 a: Mg-doped p-type Al_(0.18)Ga_(0.82)N    layer, 20 m;-   P-type cladding layer 53 b: Mg-doped p-type Al_(0.06)Ga_(0.94)N    layer, 400 nm;-   Contact layer 53 c: Mg-doped p-type GaN, 50 nm.

FIG. 6 shows an axis A_(R4) indicating the direction of the c-axis ofthe GaN supporting base. The tilt angle “C” formed between the (0001)plane of the GaN supporting base and the (0001) plane of the n-typecladding layer 43 a (the reference plane S_(R5) shown in FIG. 6) is 0.05degree or more and 2 degrees or less. The tilt angle “D” formed betweenthe (0001) plane (the reference plane S_(R6) shown in FIG. 6) of the GaNsupporting base and the (0001) planes of the light-guiding layers 49 aand 49 b is 0.05 degree or more and 2 degrees or less. The tilt angles“A” and “B” are formed in a direction opposite to each other withreference to the c-plane of the GaN supporting base. The tilt anglerelation holds over the entire primary surface of the supporting base23.

EXAMPLES

Light-emitting diodes were fabricated by organometallic vapor phaseepitaxy. FIGS. 7 and 8 are flowcharts of primary steps in a process offabricating the light-emitting diodes. The following materials wereused: trimethylgallium (TMG); trimethylaluminum (TMA); trimethylindium(TMI); ammonia (NH₃); silane (SiH₄); and bis(cyclopentadienyl)magnesium(CP₂Mg).

As shown in step S101 in a process flow 100, the following GaN waferswere prepared:

-   GaN wafer, Off-angle in m-axis direction, Off-angle in a-axis    direction.

m16: 16.4 degrees, 0.2 degree; m26: 26.4 degrees, 0.1 degree.The off-angles were determined by use of X-ray diffractometry.

The GaN wafers m16 and m26 were loaded onto a susceptor in the reactor.Semiconductor growth was carried out by the following steps: In stepS102, the wafers were heat-treated at a substrate temperature of 1050°C. under a reactor pressure of 101 kPa, while supplying NH₃ and H₂ tothe reactor. The heat treatment is used for cleaning, and the period oftime for the heat treatment was 10 minutes. Next, in step S103, NH₃,TMA, TMG and SiH₄ were supplied to the reactor to grow an AlGaN bufferlayer having a thickness of 50 nm. Then, the supply of TMA was stopped,but NH₃, TMG, and SiH₄ were supplied to the reactor without interruptionto grow a Si-doped GaN layer having a thickness of 2000 nm. AfterSi-doped GaN layer has been grown, the supply of NH₃, TMG and SiH₄ werestopped. The substrate temperature was lowered to around 700° C. NH₃,TMG, TMI, and SiH₄ were supplied to the reactor at this temperature togrow a Si-doped InGaN buffer layer having a thickness of 50 nm. Thegrowth of a light emitting layer was carried out as follows. The lightemitting layer has a three-period multiple quantum well structureincluding GaN barrier layers each having a thickness of 15 nm, and InGaNwell layers each having a thickness of 5 nm. After the quantum wellstructure was grown, the supply of TMG and TMI were stopped, and thesubstrate temperature was raised to 1000° C. TMG, TMA, NH₃, and CP₂Mgwere supplied to the reactor to grow a Mg-doped p-type AlGaN having athickness of 20 nm at this temperature. After the Mg-doped p-type AlGaNwas grown, the supply of TMA was stopped, and TMG, NH₃, and CP₂Mg weresupplied to the reactor to grow a p-type GaN layer having a thickness of50 nm. After the substrate temperature was lowered to room temperature,the epitaxial wafers were unloaded from the reactor. The structures ofthe epitaxial wafers E16 and E26 are formed from the GaN wafers m16 andm26, respectively, and these the GaN wafers m16 and m26 are the same asthe epitaxial structure of the LED shown in FIG. 5.

The photoluminescence (PL) spectra of the epitaxial wafers E16 and E26were evaluated at room temperature. A He—Cd laser of 325 nanometers wasused as excitation light source. The laser power at a sample was 1 mWand the spot diameter was about 200 μm. FIG. 9 shows typicalphotoluminescence spectra PL_(m16) and PL_(m26). The emission peakwavelength of the epitaxial wafer E16 was 500 nm, and the emission peakwavelength of the epitaxial wafer E26 was 495 nm.

In step S104, the epitaxial wafer E16 was evaluated by x-raydiffractometry. The slit size for the incident X-ray beam was 0.2 mmlong and 2 mm wide. After the off-direction was aligned to the directionof the incident x-ray beam, the height of the stage was adjusted,alignment of the axis was carried out with reference to (20-25) plane,and the offset angle of the (0002) plane was set to zero.

In step S105, the reciprocal lattice map of the (0002) plane wasprepared. FIG. 10 shows the observed reciprocal lattice map of the m16.The vertical axis indicates the inverse of the lattice constant of thec-axis multiplied by a coefficient, and the horizontal axis indicatesthe inverse of the lattice constant of the a-axis multiplied by acoefficient. The reciprocal lattice map included diffraction from theGaN substrate, diffraction from the InGaN layer, and diffraction fromthe AlGaN layer.

In step S106, the image of the reciprocal lattice map was investigated.The image of the reciprocal lattice map showed that the diffractionsfrom the InGaN layer and the AlGaN layer were not present on the ω-2θplane with reference to the GaN substrate peak. This reveals that the<0001> direction of GaN is different from the <0001> direction of InGaNand that the <0001> direction of GaN is also different from the <0001>direction of AlGaN. In step S107, the (0001) plane of the InGaN layerand the (0001) plane of the GaN layer form an angle of about 0.45degree, resulting in that these (0001) planes are not parallel to eachother. In addition, the (0001) plane of the AlGaN layer and the (0001)plane of the GaN layer form an angle of about 0.1 degree, resulting inthat these (0001) planes are not parallel to each other.

Next, the epitaxial wafer E26 was evaluated with a transmission electronmicroscope. A focused ion beam (FIB) method was used to form samples,and their damages were removed by ion milling. The incident direction ofthe electron beam was set in the <11-20> direction (the a-axisdirection) orthogonal to the off-direction angle. The accelerationvoltage of the electron beam was 200 kV. FIG. 11 is a magnified image ofan active layer and its vicinity in the LED structure of the m26 shownin FIG. 5. The transmission electron microscopic image reveals that thethree-period quantum well structure has a well width of about 5 nm. Thedislocations were not observed therein, and the high-quality activelayer was formed.

The selected-area electron diffractometry was performed to obtaininformation on lattice planes. The diameter of the selected-areaaperture was 0.1 μm. FIG. 12 shows a schematic view illustrating astructure of the epitaxial wafer E26 and measuring points associatedwith the structure. FIG. 13 shows images of diffraction patternsobserved at the three measuring points. With reference to Part (a) ofFIG. 13, the <0001> direction of the substrate tilts by about 26 degreesfrom the vertical direction to the left, as designed. With reference toPart (b) of FIG. 13, the n-GaN layer also shows a similar pattern. Thepatterns in Parts (a) and (b) of FIG. 13 reveal that the GaN layers areepitaxially grown on the respective GaN wafers. As shown in Part (c) ofFIG. 13, the observation of a region including an active layer throughselected-area electron diffractometry was performed in the same manner.FIG. 14 shows electron diffraction images at the measuring points SAD2and SAD3. FIG. 15 shows magnified views of the electron diffractionimages measured at the points SAD2 and SAD3 shown in Parts (a) and (b)of FIG. 14. With reference to Parts (a) and (b) of FIG. 15, tailsextending in a direction perpendicular to the substrate surface appearat the reciprocal lattice points in the magnified image of Part (b) ofFIG. 15 which shows the observed area for the active layer. These tailsare assigned to diffractions from InGaN and AlGaN. The occurrence of thetails shows that the <0001> direction of InGaN is not consistent withthe <0001> direction of GaN. Thus, it is clearly seen that the (0001)plane of the InGaN layer is not parallel to the (0001) plane of the GaNlayer.

Through the measurements obtained by the X-ray diffractometry and/or thetransmission electron microscopy, an amount of strain in galliumnitride-based semiconductor crystal was estimated by comparisons betweendirections of a predetermined crystal plane and a predetermined crystalaxis of GaN and directions of a predetermined crystal plane and apredetermined crystal axis of a gallium nitride-based semiconductor (forexample, InGaN, AlGaN, or AlInGaN) other than GaN in an epitaxial wafer.In step S108, it was determined whether the relaxation of strain fallswithin a desired amount range or not. If the relaxation of strain fallsin a desired level or less, the epitaxial wafer passes the test to bequalified as good items. Then, the next process of the passed epitaxialwafer was performed in step S109 for fabricating the device. Forexample, in step S110, electrodes for the semiconductor device wereformed thereon. In step S111, if the relaxation of strain exceeds thedesired level, the process for the epitaxial wafer was not carried outany more.

The above example and other experiments show that, when a tilt anglebetween the (0001) plane of the hexagonal compound of the supportingbase and the (0001) plane of the hexagonal gallium nitride-basedsemiconductor of the semiconductor layer is 2 degrees or less, therelaxation of strain is suppressed to provide a light-emitting devicewith excellent properties. A tilt angle of 0.05 degree or more canensure the verification through X-ray diffractometry.

Part (a) of FIG. 16 shows a calculated tilt angle for InGaN. The arrow“Angle” indicates the range of the off-angle. The symbol “□” indicatesthe tilt angle in the m-axis at an indium composition of 0.35, thesymbol “♦” indicates the tilt angle in the a-axis at an indiumcomposition of 0.35, and the symbol “Δ” indicates the tilt angle in thea-axis at an indium composition of 0.07. In a stack of epitaxial layersthat generates light having a peak wavelength within the range of 400 to550 nm, the indium composition is preferably in the range of, forexample, about 0.07 to about 0.35. The lower limit of the tilt angle canbe about 0.05 degree, for example, at an indium composition of 0.07 andan off-angle of 10 degrees in the a-axis. Such a value is estimated as aminimum value that can be verified by X-ray diffractometry at present.The upper limit of the tilt angle is about 1.6 degrees, for example, atan indium composition of 0.35 and an off-angle of 43 degrees in them-axis.

Part (b) of FIG. 16 shows a calculated value of a tilt angle of AlGaN.The arrow “Angle” indicates the range of the off-angle. The symbol “□”indicates the tilt angle in the m-axis at an aluminum composition of0.2, the symbol “♦” indicates the tilt angle in the a-axis at analuminum composition of 0.2, and the symbol “Δ” indicates the tilt anglein the a-axis at an aluminum composition of 0.02. In a stack ofepitaxial layers that generates light having a peak wavelength withinthe range of 400 to 550 nm, the aluminum composition is preferably inthe range of, for example, about 0.02 to about 0.2. When the tilt anglefor AlGaN is calculated in the same manner as in the case of InGaN, thelower limit of the tilt angle is about 0.005 degree, for example, at analuminum composition of 0.02 and an off-angle in the a-axis of 10degrees. Such a value is lower than the detection limit of the X-raydiffractometry. The upper limit of the tilt angle is about 0.3 degree,for example, at an aluminum composition of 0.2 and an off-angle in them-axis of 43 degrees.

The tilt angle can be calculated, for example, by the following process:(1) choose an In composition of the InGaN epitaxial film or an Alcomposition of the AlGaN epitaxial film; (2) Calculate the latticeconstant of the c-axis of a strained InGaN or AlGaN; (3) Calculate therespective angles of the (hikl) planes of the hexagonal crystal waferand the epitaxial film with reference to the (0001) plane. (4) Calculatethe angular difference between the (hikl) plane of the epitaxial filmand the (hikl) plane of the hexagonal crystal wafer. The above sequenceprovides a value of the tilt angle. In order to avoid complicatedcalculation, approximate calculation is performed by assuming that thetilt angle Δθ (the (0001) plane of the epitaxial film—the (0001) planeof the wafer) is the same as the tilt angle (the (hkil) plane of theepitaxial film—the (hkil) plane of the wafer) on the crystal plane of aplane index of an epitaxial film grown on the c-plane. Based on thisprocedure, the similar calculation can be performed for InAlGaN.

A light-emitting device in which the relaxation of strain is causedexhibits poor device characteristics, such as at least 20% decrease inluminous efficiency, an increased leakage current in low currentoperation, and a reduced lifetime. Accordingly, it is important toprevent the relaxation of strain in devices formed on semipolar planes.

With respect to the lifetime of the device in continuous currentoperation, it is noted that a sliding plane to create dislocations inthe hexagonal GaN is the (0001) plane. Since the sliding plane and thegrowing plane are parallel in the device formed on the c-plane, it ispractically impossible that the creation of dislocations occurs duringthe device operation. In contrast, since the growth plane intersectswith the (0001) plane in a device formed on a semipolar plane, thecreation of dislocations may easily occur during the device operationwhen compared to the device on the c-plane. Accordingly, it is importantto prevent the relaxation of strain in the epitaxial growth layer.

In general, the research and development of devices using a c-planesubstrate are ahead in nitride light-emitting devices. In comparisonwith the case of c-plane, significant changes in growth conditions ofepitaxial films are required in the fabrication of devices usingsemipolar planes. In the growth, in particular, the indium compositiondecreases in InGaN, whereas the aluminum composition decreases in AlGaN.In order to compensate for these decreases, when compared with thegrowth conditions on the c-plane, in the growth of InGaN, the optimumgrowth temperature range becomes lower than the temperature of thecrystal growth on the c-plane by about 50 to 150 degrees (Celsiustemperature unit). In the growth of AlGaN, as compared with the growthcondition for the c-plane, the optimum growth temperature range becomeshigher than the temperature of the crystal growth on the c-plane byabout 10 to 50 degrees (Celsius temperature unit). The temperaturesequence for growing a structure for a light-emitting device greatlydiffers between the c-plane and the semipolar plane.

Having described and illustrated the principle of the invention in apreferred embodiment thereof, it is appreciated by those having skill inthe art that the invention can be modified in arrangement and detailwithout departing from such principles. We therefore claim allmodifications and variations coming within the spirit and scope of thefollowing claims.

INDUSTRIAL APPLICABILITY

As described above, one aspect of the present invention provides a groupIII nitride semiconductor device that can suppress the occurrence ofdislocations caused by the relaxation of strain in a galliumnitride-based semiconductor utilizing semipolar. Another aspect of thepresent invention provides an epitaxial wafer for the group III nitridesemiconductor device.

REFERENCE SIGNS LIST

-   C: c-axis direction of hexagonal compound;-   SSUB_(C): c-plane of hexagonal compound;-   SLAY_(C): c-plane of hexagonal gallium nitride-based semiconductor;-   DSUB_(N): lattice constant of hexagonal compound of polar plane    wafer;-   DLAY_(N1): lattice constant of hexagonal gallium nitride-based    semiconductor;-   DSUB_(P): lattice constant of hexagonal compound of nonpolar plane    wafer;-   DLAY_(P1) and DLAY_(P2): lattice constant of hexagonal gallium    nitride-based semiconductor;-   DSUB_(H): lattice constant of hexagonal compound of semipolar plane    wafer;-   DLAY_(H1), DLAY_(H2): lattice constant of hexagonal gallium    nitride-based semiconductor;-   SSUB_(C): c-plane of wafer;-   SLAY_(C1), SLAY_(C2): c-plane of hexagonal gallium nitride-based    semiconductor;    -   S_(R1), S_(R2): reference plane;-   α: tilt angle between c-plane SLAY_(C1) and reference plane S_(R1);-   A_(R): auxiliary line;-   β: angle between axis A_(R) and vector C;-   γ: tilt angle between c-plane SLAY_(C1) and reference plane S_(R2);-   δ: angle between auxiliary line A_(R) and vector C;    -   11 a, 11 b, 11 c: wafer composed of hexagonal compound;-   13 a, 13 b, and 13 c: semiconductor layer composed of hexagonal    gallium nitride-based semiconductor;-   15 a, 15 b, and 15 c: hexagonal gallium nitride-based semiconductor;-   17 a, 17 b, and 17 c: semiconductor layer composed of hexagonal    gallium nitride-based semiconductor;-   19 a, 19 b, and 19 c: hexagonal gallium nitride-based semiconductor;-   21 a: light-emitting diode;-   21 b: semiconductor laser;-   23: supporting base;-   25 a, 25 b: semiconductor stack;-   27: active layer;-   29: n-type gallium nitride-based semiconductor region;-   31: p-type gallium nitride-based semiconductor region;-   33 a: buffer layer;-   33 b: n-type GaN layer;-   35 a: electron-blocking layer;-   35 b: contact layer;-   37 a: well layer;-   37 b: barrier layer;-   41: n-type gallium nitride-based semiconductor region;-   43: n-type cladding layer;-   45: optical waveguide region;-   47 a: active layer;-   47 b: n-side optical guiding layer;-   47 c: p-side optical guiding layer;-   49 a: well layer;-   49 b: barrier layer;-   51: p-type gallium nitride-based semiconductor region;-   53 a: electron-blocking layer;-   53 b: p-type cladding layer;-   53 c: contact layer.

1. A group III nitride semiconductor device, comprising: a supportingbase having a primary surface, the primary surface comprising ahexagonal compound, the primary surface tilting by an off-angle of 10degrees or more and less than 80 degrees with reference to a c-plane ofthe hexagonal compound; and a semiconductor region provided on theprimary surface of the supporting base, the semiconductor regioncomprising a semiconductor layer, the semiconductor layer comprising ahexagonal gallium nitride-based semiconductor different from thehexagonal compound, a tilt angle between a (0001) plane of the hexagonalcompound of the supporting base and a (0001) plane of the hexagonalgallium nitride-based semiconductor of the semiconductor layer being ina range of +0.05 degree to +2 degrees and −0.05 degree to −2 degrees,and the hexagonal gallium nitride-based semiconductor of thesemiconductor layer comprising one of AlGaN and InGaN. 2.-23. (canceled)